I was thinking about getting one. I was looking on Ebay and noticed there are several different systems. What do I need to look for in a system, or does it not matter / they are all basically the same ?

We have been using the Kent 'Bare Bones' unit for approx. 7 years, and have been very satisfied with it. I believe it is rated at 12 gals. per day. This system offers two different types of membranes, based on the type of water you will be filtering.

A lot of the systems on EBAY are people who buy the parts from overseas (asia) and then put together an RO system. Some of these parts are cheaply made (I bought an eBay RO system a few years ago, it leaks in two places despite all my efforts and I broke two elbow pieces putting it together).

i have the fancy-pants kent and really like two of the features. the first is the pressure gauge so you can pull water at an ideal rate for filter function. the second is the ppm meter which gauges intake and output. i had a membrane rupture and would have not realized it without the ppm meter, and was also able to tell when the DI resin wore out.

check buckeyefieldsupply.com and thefilterguys.biz. IMO you dont really need to go as far as having a DI filter on it. 3 stage filter is fine for making water for your misters and what not. If you don't need massive amounts of RO water daily you may want to get one of the home drinking water systems that come with 3-4 gallon holding tank so you can also have good water to drink for yourself.

The main difference between the hobby units and the consumer units sold in the box stores is the cost of replacement cartridges. The hobby units tend to all use standardized replacement cartridges available from a variety of vendors (and there's almost always a sale on somewhere) and can have a substantially lower cost of ownership than the consumer units that use proprietary cartridges.

The other difference is that you can change up the filtration to suit your conditions -- around here, all of our drinking water is surface water from local lake reservoirs, and in the summer (when the blue-green algae really kicks in) the water department occasionally turns up the chloramines used for disinfectants to problematic levels. I had trouble with my reef tanks in the summers until I figured out what was going on and started using specialty "chloramine-buster" cartridges, because the standard ones weren't removing it all.

I currently have two units (one in the fish room, the other in the kitchen for cooking and to support a display tank at that end of the house), one from AirWaterIce and the other from Bulk Reef Supply; there's essentially no difference between them, other than who had the best deal at the time. I tend to buy most of my replacement cartridges from Bulk Reef Supply; I've also had good support from them when working through issues with parts, etc.